Investigating local charge- and vacancy-ordering in a sodium-ion cathode

Sodium-ion batteries (NIBs) are a cheaper alternative to lithium-ion batteries. At present, the capacity of NIBs is too low for practical applications and limited by the cathode; to increase this capacity, a detailed understanding of the charge compensation mechanism is required. We have been investigating a new cathode material, Na0.67[Mg0.28Mn0.72]O2, with a promisingly high capacity. Bulk structural invesitgations - using laboratory X-ray diffraction - reveal honeycomb ordering across the Mn/Mg sublattice. However, fits to the data using a perfectly ordered structure are poor. Local structural investigations, using 23Na, 25Mg and 17O NMR, have also been carried out. Assignment of these spectra using DFT calculations only partially account for the spectra. A more detailed structural model is required; we aim to create such a model from neutron diffraction and total scattering data.